1. Field of the Invention
The present invention is directed to an automatic dilution system. Specifically, the present invention provides for an automatic dilution system which ideally provides for an exponential dilution to an appropriate concentration.
2. Description of the Prior Art
The physical or chemical analysis of various types of fluid samples is often accomplished by diluting the sample with a diluent to an appropriate concentration. The fluid samples may be any of a wide variety of solutions, suspensions, and dispersions. As used in the present application, the term "diluent" refers to either a gas or liquid dependent upon whether the dilution application requires the sample to be diluted in a dry or liquid state.
In certain measurement applications, the dilution of the fluid sample is carried out using a known dilution factor. This known dilution factor is either predetermined at the onset of the measurement or is computed after the dilution has been accomplished. In the prior art, a variety of methods and devices have been developed to achieve a known or predetermined dilution of a fluid sample. These prior art methods and devices have been limited in their utility and cannot provide for a variable dilution when such a variable dilution would be more useful than the known or predetermined dilution of the prior art devices.
As an example, the following prior art patents may be pertinent to the present invention. Cruzan U.S. Pat. No. 4,036,062, Roof et al U.S. Pat. No. 4,036,063 and Roof U.S. Pat. No. 4,070,913 all describe means for diluting a liquid sample with liquid diluent in which each of the two fluids is initially contained in a pair of conduits. The two conduits are connected together to permit a closed loop circulation and mixing of the two fluids. The extent of the dilution is determined at the onset by preselecting the volumetric relationship of the two conduits.
The Mowery, Jr. U.S. Pat. No. 4,095,472 describes a system wherein a liquid sample is diluted by directing independent streams of a sample liquid and a liquid diluent each at a constant preset flow rate into a mixing chamber. The diluted sample fluid can then be extracted from the mixing chamber. In this patent a fixed dilution factor is established at the onset. The Culbertson U.S. Pat. No. 3,805,831 describes a mixing apparatus for continuously and proportionally mixing one fluid stream with another. The final sample concentration which emerges is determined by the composition of each stream and their relative rates of flow.
The Pardikes U.S. Pat. No. 4,279,759 describes an optical sensing device to measure the presence of a treatment chemical in a liquid process stream. This patent also controls, by negative feedback, the rate of introduction of the treatment chemical into the continuously flowing stream so as to establish a relatively fixed concentration of the treatment chemical in the stream. Moreaud et al U.S. Pat. No. 4,348,112, Tsuji et al U.S. Pat. No. 4,408,880 and Brenholdt U.S. Pat. No. 4,507,556 describe various sensor techniques based on light scattering and/or defraction to estimate either the particle value or particle concentration in a liquid suspension.
It can be seen, therefore, that a variety of methods and devices exist in the prior art to achieve known or predetermined dilutions of a sample fluid. However, there are other types of measurements of physical or chemical properties of fluid samples wherein the measurement is more properly accomplished by diluting the fluid sample to an extent which is not predetermined at the outset of the dilution process. In these types of measurements, the final extent of dilution may be controlled by some measurable property of the fluid sample which property changes considerably during the dilution process. For example, the measurable property of the fluid sample may be optical turbidity, color, electrical conductivity, pH, etc. The prior art methods and devices cannot provide for this variable dilution which changes during the measurement process in accordance with the change in some measurable property of the fluid sample. The prior art methods and devices are limited in their utility for this type of system.
There are a large number of commercial products which contain fine particles which exist either in a dry state or suspended in an appropriate solvent such as water. The physical and/or chemical properties of these commercial products usually depend significantly on the distribution of particle sizes or molecular weights of the individual particles or molecules contained in the product. Typically, when liquid samples are obtained in a manufacturing process, these samples contain a high concentration of solute particles or macromolecules often exceeding 10% concentration by weight or volume. However, most analytical instruments are designed to measure particle size or molecular weight only if provided with a sample in the form of a dispersion of particles in gas or liquid which is much less concentrated than the concentration normally obtained at the outset from the manufacturing process.
Therefore, there is usually the need to perform a substantial dilution of the original sample. This dilution would normally be accomplished using a fluid diluent which is either a gas or a liquid. For this type of application and for others it would be desirable to develop a simple dilution apparatus which yields an acceptable final dilution of a fluid sample which is appropriate or optimal for the analytical measurement in question. However, the dilution apparatus must ideally operate without any knowledge of the starting concentration or composition of the particular sample, whether in a dry state or in liquid suspension.
In the prior art, dilutions are normally achieved by measuring out a known volume of a starting fluid sample into a suitable container and adding to this, either simultaneously or subsequently, a known volume or amount of diluent. The resulting mixture is then thoroughly mixed so as to disperse the solute particles from the original sample uniformally within the new fluid volume. The result ideally is a new fluid mixture or suspension which is homogeneous and has a lower concentration of the solute component then the original fluid sample at the onset of the dilution process.
As an example, U.S. Pat. Nos. 4,036,062, 4,036,063 and 4,070,913 describe methods of carrying out such a fixed dilution. However, this traditional approach to dilution is inconvenient and relatively inaccurate when large dilution factors are desired. In these situations it is difficult to meter out accurately a very small volume of starting sample fluid to be then added to a given amount of diluent. To overcome this problem it may be necessary to perform multiple dilutions in series in which each individual dilution factor is relatively small and, therefore, accurately controllable. The final dilution factor is then equal to the product of the individual ones. However, such an apparatus is necessarily more complex and more difficult to maintain because of the larger number of individual stages.
In order to perform an analytical measurement a quantity of the new diluted fluid sample is transferred from the mixing container into the appropriate measuring instrument. This transfer is normally provided either by manual means, such as pipetting, or by means of an automatic fluid handling system. Unfortunately, for most analytical instruments the straightforward method of diluting the fluid sample as described above is not very efficient; rather, the dilution factor must often be adjusted in a trial-and-error fashion in order to obtain a final dilution factor which results in optimal performance of the analytical instrument. For example, the initial dilution of the original fluid sample may be insufficient thereby resulting in an overloading or saturation of the measuring instrument. Alternatively, the dilution of the original fluid sample may be too extensive thereby yielding an inaccurate measured signal.
Automatic dilution systems have also been developed which continuously introduce both the starting sample and diluent fluid into the mixing chamber. The input rates of each of these components can be adjusted to fixed known values so as to yield a final diluted fluid sample whose dilution factor remains known. The dilution factor may also be constant in time as some of the final fluid sample is removed from the mixing valume. These systems permit, at least in principle, the dilution factor to be preset to any practical desired value to thereby result in a final solute concentration ranging from a very low value to a very high value of concentration. This type of adjustable dilution system may be seen with reference to U.S. Pat. No. 4,095,472.
Automatic dilution systems have also been developed which rely on the principle of negative feedback. In these systems, one or both of the flow rates of the original sample and diluent into the mixing chamber are continuously adjusted by a mechanism which responds to some measurement of the resulting diluted fluid sample. Typical measurements include turbidity, optical absorbance at a particular wavelength and light scattering intensity, all of which are representative of the solute concentration. The measurement which changes with the concentration of solute particles in the diluent fluid sample is used to automatically adjust the dilutor mechanism so as to yield an approximately unchanging final solute concentration. Such a system is described in U.S. Pat. No. 4,279,759. This type of more sophisticated dilution system is actually an adjustable version of the fixed dilution system described above. However, because of the principle of negative feedback the final solute concentration is kept approximately constant in time with the arrival of addtional sample and diluent. The above described prior art automatic dilution systems provide a background for the automatic dilution system of the present invention which provides for an infinitly variable dilution of a starting fluid sample.